1. Field of the Invention
The present invention is related to an organic EL display and a method for manufacturing an organic EL display. Especially, the present invention is related to an organic EL display and a method for manufacturing an organic EL display, in which luminance degradation and life-time reduction are controlled by forming finally a blue polymer organic EL light emitting material because luminance degradation and life-time reduction of blue color polymer organic EL light emitting material in air is remarkable.
2. Description of the Related Art
An organic EL element emits light when a current is supplied to a light emitting layer which is formed from an organic light emitting material and between two opposed electrodes, and, in order to achieve efficient light emission, it is important to keep a film thickness of the organic light emitting layer to about 100 nm. Further, in the case of forming a display from the organic EL element, it is necessary to perform patterning of the organic EL element with high definition.
As the organic light emitting material for the light emitting layer formed on a substrate or the like, a low molecular material and a high molecular (polymer) material are usable. The low molecular material is subjected to resistive heating vapor deposition and the like to form a thin film, and at the same time the patterning is performed by using a microscopically patterned mask. However, this method has a problem that patterning accuracy is reduced with an increase in size of a substrate.
Therefore, the high molecular material has recently been used as the organic light emitting material, and a method of forming a thin film by wet coating with a coating liquid obtained by dispersing or dissolving the organic light emitting material into a solvent has been tried.
As the wet coating method for the thin film formation, spin coating, bar coating, projection coating, dip coating, and the like are known. However, the wet coating methods have difficulty in realizing the high definition patterning and color coding with R, G, and B, and it is considered that the thin film is most effectively formed by a printing process that is capable of achieving excellent color coding and patterning.
It is often that an organic electroluminescence element or a display uses a glass substrate as a substrate. Thus, a method to use a metal hard printing plate such as a gravure printing method is unsuitable. Offset printing to use rubber blanket having elasticity and relief printing method to use rubber printing plate and photosensitive resin printing plate having elasticity are suitable. As attempt by these printing methods, a method (Japanese Patent Laid-Open No. 2001-93668 Official Gazette) by offset printing and a method (Japanese Patent Laid-Open No. 2001-155858 Official Gazette) by relief printing are really proposed.
It is known that there is the most suitable viscosity in an ink of a viscous type (in other word, thixotropy type) or a liquid type used for a relief printing method. Especially, a viscosity modifier such as a thickener or a surface activating agent for adjusting a surface tension is generally added to a liquid type ink.
In the case where electronic materials are printed, solubility thereof may be limited or impurity may be not preferably included therein.
Especially, when a film is formed by printing an organic light emitting material using a printing method, an organic light emitting material is dispersed or dissolved in a solvent such as a water, an alcohol or an organic solvent (including a binder resin if necessary), thereby an ink for printing or coating is made.
It is said that, in a case where a pattern of a film of an organic light emitting material is formed and the pattern is made to emit light in a element, the higher the purity of a film made of an organic light emitting material is, the better the durability of an element is. Therefore, since the remained thickener or the like lowers the purity, the thickener or the like can not be included in an ink. Therefore, from this reason, some adjustable properties of an organic light emitting material ink are limited, the some adjustable properties being for achieving an ink transfer property of a printed matter and achieving stability of a pattern shape.
From the above-mentioned reason and low solubility of especially light emitting material case, only some kinds of aromatic solvents can be used, thereby range of choice in inks is not so broad.
In addition, in the case where a light emitting material film is only exposed to air after the film formation, the film may be degraded while intensity of emitted light may decreased. Especially, as for generally used polymer organic EL light emitting material, life-time thereof in the case the material is used for a display is not sufficient for an assured performance. However, this fact depends on a kind of a display.
Further, under such a situation, if blue color materials are compared with red or green color materials, blue color materials has essentially shorter life-time. Further, there is a problem in which blue color materials are remarkably degraded by outside light or air.
The object of the present invention is to provide an organic EL display and a method for manufacturing an organic EL display. In this organic EL display, luminous degradation and reduction of life-time can be control led by forming last a blue polymer organic light emitting material pattern last since luminous and life-time of a blue polymer organic light emitting material are remarkably degraded in air.    [Patent document 2] JP-A-2001-93668    [Patent document 3] JP-A-2001-155858    [Patent document 1] JP-A-2001-155861